Many types of printing systems include one or more printheads that have arrays of marking elements that are controlled to make marks of particular sizes, colors and densities in particular locations on the print media in order to print the desired image. In some types of printing systems, the array of marking elements extends across the width of the page, and the image can be printed one line at a time. However, the cost of a printhead that includes a page-width array of marking elements is too high for some types of printing applications, so a carriage printing architecture is often used.
In a carriage printing system such as a desktop printer, or a large area plotter, the printhead or printheads are mounted on a carriage that is moved past the recording medium in a carriage scan direction as the marking elements are actuated to make a swath of dots. At the end of the swath, the carriage is stopped, printing is temporarily halted and the recording medium is advanced. Then another swath is printed, so that the image is formed swath by swath. In a carriage printer, the marking element arrays are typically disposed along an array direction that is substantially parallel to the media advance direction, and substantially perpendicular to the carriage scan direction. The length of the marking element array determines the maximum swath height that can be used to print an image.
In an inkjet printer, the marking elements are drop ejectors, where each drop ejector includes a nozzle and a drop forming mechanism, such as a bubble-nucleating heater. Some carriage printers have more than one drop ejector array for printing a particular ink. This enables faster printing throughput because within a swath some dots are printed by one drop ejector array and some dots are printed by another drop ejector array. The carriage velocity is therefore not limited by the maximum refill frequency of a single drop ejector. In addition, by having some dots printed by two different drop ejector arrays in a single pass, printing defects from either drop ejector array are disguised by the dots that are printed by the other drop ejector array. For example, if drops from a particular drop ejector are misdirected in a first drop ejector array there could be a white line in an image if only that drop ejector array were used to print in a single pass. By using two different drop ejector arrays, dots from a corresponding drop ejector of the other drop ejector array can partially fill in the white line, and disguise the defect somewhat. In other words, good image quality can be provided in fewer multiple printing passes if there is more than one drop ejector array for a particular ink.
Faster printing throughput can also be achieved by printing at a faster carriage speed. However, the distance d required to accelerate from a stopped position to a constant velocity vc is given by d=vc2/2a, where a is the acceleration. Therefore, as the carriage velocity is increased, it is desirable to increase the acceleration so that the width of the acceleration region doesn't increase to unacceptable levels, requiring that the printer be significantly wider than the print media. In order to further increase printing throughput, some printers print during acceleration or deceleration. However, acceleration and deceleration of the carriage can cause ink pressure changes that can result in image quality degradation under certain circumstances, particularly for large magnitudes of acceleration or deceleration.
Although the use of two drop ejector arrays to print dots of a particular ink can provide increased printing throughput by sharing the printing responsibilities in printing regions where there is substantially constant carriage velocity or low levels of acceleration, it would be advantageous to enable further increases in printing throughput by printing at increased levels of acceleration, while providing excellent image quality.